Methods for the production of structured layers based on heat-resistant polymers are known, for example, from German Pat. No. 2 308 830 and from the European Patents 0 019 123 and 0 026 820. In these processes, soluble photoreactive precursors of highly heat resistant polymers are employed for photolithographic structuration, and in a subsequent tempering step the structures produced therefrom are cyclicized to highly heat resistant structures. For complete cyclization and removal of the cleavage products, temperatures up to 400.degree. C. are needed. This requires substrates capable of high thermal load.
In circuit and transmission applications, e.g., in circuitboard technology, epoxy-based substrates which can be subjected to a thermal load of up to about 150.degree. C. per hour and which can withstand temperatures of about 260.degree. C. for a number of seconds in soldering processes are employed. The solder resists employed for partial conductor run coverings must meet similar thermal requirements; in order to cover the areas of the circuit surface which are not in contact with the solder metal, polymers of a medium thermal stability are needed. The epoxy and acrylate-based dry resists and screen printing lacquers which are still used for this purpose meet the requirements of a solder stop mask, yet these materials only partially fulfill the more stringent requirements for dimensional accuracy needed in micro-conductor technology with structures of less than 100 um, and at the stipulated cycle strength. For this, photolithographic lacquer systems are needed. There is already available a photostructurable epoxy-based lacquer system, which has chalcone groups incorporated in the polymer chain, and which provides sufficient dimensional accuracy. However, relatively long exposure and developing times are necessary for photostructuring. Moreover, the circuit system must often be protected from noxious gas; with the known lacquer system such protection can only be obtained by using costly multiple coatings. Because hardening times of several hours are required, the process is lengthy and expensive.
It is an object of the invention to provide a method for producing dimensionally exact, high-grade structured layers, for circuit surfaces in particular, which will withstand the extensive thermal and mechanical stresses of, for example, dip soldering, and which effectively and durably protect the circuit surface against moisture and corrosion with one coating process. It is another object to provide a method for producing structured layers, wherein the exposure, developing, and tempering times are shortened, so that the method is cost-efficient.